The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An aircraft is driven by one or several propulsion units each comprising a turbojet engine housed in a tubular nacelle. Each propulsion unit is fastened to the aircraft by a mast which is generally located under or above a wing or at the fuselage.
By upstream, it is referred to everything that lies before the considered point or element, in the direction of the air flow in a turbojet engine, and by downstream, it is referred to everything that lies after the considered point or element, in the direction of the air flow in a turbojet engine.
A nacelle generally has a structure comprising an air inlet upstream of the engine, a mid-section intended to surround a fan or the compressors of the turbojet engine and its casing, a downstream section able to integrate thrust reversal means and intended to surround the gas turbine of the turbojet engine, and is generally terminated by an ejection nozzle the outlet of which is located downstream of the turbojet engine.
Conventionally, the space that it is comprised between the nacelle and the turbojet engine is called the secondary duct.
In general, the turbojet engine comprises a set of blades (the compressor, and possibly a fan or an unshrouded propeller) which are driven in rotation by a gas turbine via a set of mechanical transmission means.
A lubricant distribution system is provided in order to provide good lubrication of these transmission means and any other accessory such as the electric generators, and to cool them.
As a consequence, the lubricant must also be cooled by a heat exchanger.
To this end, a first known method consists of cooling the lubricant by conveying it through an air/oil heat exchanger which uses air that is collected in a secondary duct (so-called the cold stream) of the nacelle or in either one of the compressor stages.
Collecting and conveying air through this heat exchanger disturbs the air flow stream and results in a loss of thrust of the engine, which is not desired.
In particular, it has been assessed that, in the case of a geared turbofan engine, this may represent losses that are equivalent to 1% of the total fuel consumption.
Another solution has appeared with the nacelle deicing systems.
In fact, during flight and depending on the conditions of temperature and humidity, ice may form on the nacelle, in particular at the outer surface of the air inlet lip which equips the air inlet section.
The presence of ice or frost modifies the aerodynamic properties of the air inlet and disturbs the delivery of air toward the fan. In addition, the formation of frost on the air inlet of the nacelle and the ingestion of ice by the engine in the case where the ice blocks are detached may damage the engine or the wings, and pose a risk to the safety of the flight.
Another solution for defrosting the outer surface of the nacelle consists in preventing ice from forming on this outer surface by maintaining the considered surface at a sufficient temperature.
Thus, the lubricant heat may be used to reheat the outer surfaces of the nacelle, and consequently, the lubricant is cooled and ready to be used again in the lubrication circuit.
In particular, the documents U.S. Pat. No. 4,782,658 and EP1479889 describe the implementation of such defrosting systems which use the heat of the engine lubricant.
More specifically, the document U.S. Pat. No. 4,782,658 describes a defrosting system which uses external air that is collected by an air scoop and reheated through an air/oil heat exchanger in order to serve for defrosting. Such a system allows for a better control of the exchanged heat flows, but the presence of air scoops at the outer surface of the nacelle results in a loss of the aerodynamic performances.
As for the document EP1479889 it describes, a system for defrosting an air inlet structure of a nacelle of a turbojet engine which uses an air/oil heat exchanger in a closed circuit, the reheated internal air of the air inlet structure being driven in forced convection by a blower.
It is worth noting that the air inlet structure is hollow and it forms a closed chamber intended for conveying the defrosting air which is reheated by the heat exchanger disposed inside this chamber.
Thus, the heat energy that is available for defrosting depends on the temperature of the lubricant.
Furthermore, the exchange surface of the air inlet structure is stationary and limited, and the energy that is actually dissipated depends mainly on the heat that is required for defrosting, and hence, it is dependent on the external conditions.
It follows that the lubricant cooling, as well as the temperature at which the air inlet is maintained, are difficult to control.
There is another solution wherein a heat exchanger and ducts for circulating a fluid to be reheated are associated so as to form several loops for recirculating the fluid to be reheated through the heat exchanger, and in such a manner that an area intended for circulating the fluid to be reheated is in contact with an outer wall so as to allow a conductive heat exchange with the external air of the nacelle. The circulation of the fluid to be reheated is achieved by forced circulation.
It has been observed that systems such as those described before cause pressure drops in the secondary duct due to the presence of the heat exchanger, and cause engine thrust losses when an air collection is performed in the secondary duct where these losses considerably affect the consumption (they represent about 0.5% of the total consumption). Furthermore, such systems have a poor efficiency when the turbojet engine idles and/or rotates at low speed (for example, when the aircraft is on the ground, during the taxiing phase) in the case where cooling the engine oil requires collecting air from the outside of the nacelle.